The present invention relates to integrated circuits and packaged integrated circuits and, more particularly, to a leadframe for an integrated circuit.
An integrated circuit (IC) die is a small device formed on a semiconductor wafer, such as a silicon wafer. A leadframe is a metal frame that usually includes a paddle that supports an IC die that has been cut from the wafer. The leadframe has lead fingers that provide external electrical connections. That is, the die is attached to the die paddle and then bonding pads of the die are connected to the lead fingers via wire bonding or flip chip bumping to provide the external electrical connections. A package is formed by encapsulating the die and wire bonds or flip chip bumps with a protective material, such as a mold compound. Depending on the package type, the external electrical connections may be used as-is, such as in a Thin Small Outline Package (TSOP) or quad-flat no-lead (QFN), or further processed, such as by attaching spherical solder balls for a Ball Grid Array (BGA). These terminal points allow the die to be electrically connected with other circuits, such as on a printed circuit board.
Referring to FIG. 1, a top plan view of a leadframe 10 having lead fingers 12 and a large, solid die paddle 14 is shown. The lead fingers 12 have inner lead portions 16 proximate to the die paddle 14 and outer lead portions 18. The outer lead portions 18 are connected together with a tie bar 20 that is spaced from and encircles the die paddle 14. The corners of the die paddle 14 are connected to the tie bar 20 with support pins 22. An IC die (not shown) that would be attached to the die paddle 14 would have a footprint, shown in dashed lines, that is generally smaller than that of the die paddle 14. Leadframes are typically made of copper, a copper alloy, or an iron-nickel alloy.
Package cracking is a common problem in plastic encapsulated semiconductor devices. Such cracking arises from a combination of factors. One factor is internal delamination between the plastic encapsulant material and the die paddle due to the different coefficients of thermal expansion (CTE) of the metal leadframe and the plastic encapsulant. As a result of this CTE mismatch, stress is created at the plastic-die paddle interface as the semiconductor device experiences temperature changes. The stress is relieved through delamination of the plastic-die paddle interface. Another cause of package cracking is moisture absorption. After the plastic-die paddle interface becomes delaminated, moisture from the environment diffuses through the molding compound to the delaminated area. Once moisture accumulates in the package, rapid temperature increases will cause the moisture to vaporize and expand, thereby creating an internal pressure pocket in the delaminated area. The surrounding plastic cracks in order to relieve the pressure at the pressure pocket. Such delamination and cracking often occur during a solder reflow process, where the semiconductor device experiences a rapid increase in temperature.
Numerous methods are available for dealing with the problem of package cracking. One method is dry packing, which involves baking plastic encapsulated devices sufficiently to reduce moisture content and packaging the devices into moisture resistant packets. The device is then attached before a sufficient amount of moisture to cause cracking can be absorbed into the package through exposure to ambient conditions. This method, however, adds an additional step to the assembly process and requires users to keep track of how long a device is exposed to ambient conditions. Other methods of reducing delamination and cracking involve improving adhesion between the die and the die paddle, such as by roughening the die paddle, forming holes or dimples in the die paddle, and the use of a window-frame type die paddle, in which a large area is cut from the die paddle.
FIG. 2 shows another leadframe design that reduces delamination and plastic cracking. The leadframe 30 has lead fingers 32 and an X-shaped die paddle 34. Like the lead fingers 12 shown in FIG. 1, the lead fingers 32 have inner lead portions and outer lead portions, and a tie bar 36 that connects the lead fingers 32. The corners of the X-shaped die paddle 34 are connected to the tie bar 36 with support pins 38. The support pins 38 are thinner than the tie bar 36. An IC die (not shown) that would be attached to the die paddle 34 would have a footprint as shown in dashed lines. Note that portions of the IC die would not be in contact with the die paddle 34 when attached thereto. Note also that the die paddle 34 can accommodate various size IC die and still perform the same function and achieve the same goals, i.e., reduced delamination and cracking. The X-shaped flag design improves adhesion between the lead frame and the encapsulating material by also providing a die-plastic (encapsulating material) interface.
While the X-shaped die flag design has a high moisture sensitivity level performance, some devices, such as power transistors, require high thermal performance. That is, it is preferred that such devices include a heat sink. The present invention provides a leadframe for a semiconductor device with good thermal performance, high moisture sensitivity level performance, and can be used with various die sizes.